High support double layer forming fabric

ABSTRACT

A double layer forming fabric for a paper making machine is woven to an overall repeating pattern, which comprises in combination paper side layer weft yarns, machine side layer weft yarns, and warp yarns. The paper side layer weft yarns interweave with the warp yarns in a first repeating weave pattern, the machine side layer weft yarns interlace with the warp yarns in a second repeating weave pattern, and the fabric is woven in an overall repeating pattern requiring 3N sheds, in which N is an integer and is at least 2. For each warp yarn, the first repeating weave pattern includes interweaving locations comprising a first and a second interweaving point, each of the interweaving points being separated by an internal warp float of at least two paper side layer weft yarns. For each warp yarn, the second repeating weave pattern includes a series of interlacing points in which each interlaced machine side layer weft yarn also passes substantially below an interweaving location of paper side layer weft yarns and each adjacent warp yarn. In addition, for each warp yarn, each of the first and second interweaving points with a paper side layer weft yarn is separated from an adjacent machine side layer interlacing point by at least two machine side layer weft yarns. The machine side face of the machine side layer includes exposed machine side layer weft yarn floats having a float length L defined as L=3N-M, wherein M is an integer and is at least 1.

FIELD OF THE INVENTION

[0001] This invention relates to a double layer forming fabric,consisting of a single set of warp yarns, and two layers of weft yarns,for use in the forming section of a paper making machine.

BACKGROUND OF THE INVENTION

[0002] In the forming section of a paper making machine, an aqueousstock is deposited onto the paper side surface of a moving formingfabric. The machine side surface of the forming fabric is in contactwith the static fabric support elements in the forming section of thepaper making machine. The forming fabric allows water to drain from thestock, and retains a proportion of the paper making solids in the stockon its surface to form an incipient paper web.

[0003] It has been found that the desirable characteristics for suchfabrics are to a degree mutually incompatible, both in achieving anacceptable balance between the drainage, formation and retentionproperties of the fabric, and in other factors affecting the selectionof weave patterns to achieve optimal properties for the paper side andthe machine side of the forming fabric. The forming fabric must becapable of withstanding the mechanical and abrasive stresses imposed onit, which, in modern paper making machines where the forming fabricmoves at a speed in excess of 70 kph, are substantial. To produceacceptable quality paper, the forming fabric should not cause marking,known as wire mark, on the sheet, and the percentage of the paper solidsin the stock retained in the incipient paper web, known as first passretention, should be as high as possible. In order to achieve a highfirst pass retention, the forming fabric must have good drainagecharacteristics and low water carrying properties, so that the removedwater is readily transported through the fabric, without excessivedrainage and loss of the paper solids. The fabric should also provide auniform, planar support surface onto which the stock is delivered sothat the paper making fibres are evenly supported by the component yarnsof the paper side surface and the resulting sheet does not exhibitsubstantial variation in its fibre distribution and is stated to be“well-formed”. In addition, as a significant proportion of the fibres inthe stock delivered onto the moving forming fabric tend to be orientedin the machine direction of the forming fabric, the fabric shouldprovide adequate fibre support in the cross machine direction.

[0004] The need for a high drainage rate calls for a fabric with an openweave, but such a weave tends to cause wire mark and the incipient paperweb tends to be formed somewhat in, rather than mostly on, the formingfabric paper side surface. A closely woven fabric provides better papersupport and results in good first pass retention, and the paper isformed on, rather than somewhat in, the fabric, and is thus easier torelease from the forming fabric. However, a closely woven fabric drainsrelatively poorly.

[0005] It has been found that improved drainage and fibre supportcharacteristics can be achieved by ensuring that the frame openings inthe paper side layer are substantially regular, and if the openings arerectangular, it is preferable that the longer side be oriented in thecross-machine direction. However, a related factor is the undesirableeffect of forces which tend to induce adjacent pairs of weft yarns tomove closer together, creating an asymmetry, known as “twinning”. Thisreduces alignment and registration of the paper side and machine sideyarns, and the resulting different sized drainage passages adverselyaffect paper quality. Various methods have been suggested to resolvethis problem.

[0006] Wilson, in U.S. Pat. No. 6,112,774, suggests that twinningresults from excessive tension in machine direction yarns where thoseyarns interlace in the machine side layer with the cross-machinedirection yarns in an “under 1, over 1, under 1” configuration, forexample in the zig-zag -machine side layer weave pattern disclosed byWright in U.S. Pat. No. 5,025,839. Wilson discloses a weave patternwhich maintains the zig-zag pattern of Wright for the machine sidelayer, but suggests an arrangement of alternating machine directionyarns, in which the machine side layer interlacing points on adjacentmachine direction yarns are offset by at least two cross machinedirection yarns, as a means of reducing tension in the machine directionyarns.

[0007] Nevertheless, it has been found that twinning of paper side layerweft yarns continues to occur in weave patterns such as disclosed byWilson in U.S. Pat. No. 6,112,774. Wilson further suggests, in WO01/59208, that cross-machine direction yarns can be maintained in theiroriginal positions, i.e. that twinning can be reduced, by the use ofsuggested preferred materials for the manufacture of the machinedirection yarns. These materials are said to encourage crimping,particularly where the machine direction yarns interweave with the crossmachine direction yarns in an “over 1, under 1, over 1, under 1, over 1”configuration.

[0008] However, it has recently been found that twinning of paper sidelayer weft yarns adjacent to interweaving points in that layer can beavoided, or significantly reduced, in a double layer fabric, by usingweave patterns which do not involve the close proximity of interweavingpoints on adjacent paper side layer warp yarns. This advantage isfurther enhanced where the weave pattern additionally does not involvethe close proximity on a single warp yarn of the last one of a series ofinterweaving points in the paper side layer and an immediately adjacentinterlacing point in the machine side layer. It has thus been found thatthe undesirable twinning effect can be significantly reduced byproviding a weave pattern which maximizes the distance betweeninterweaving points in the paper side layer on adjacent warp yarns,while increasing the internal float length of the warp yarns between theinterweaving points on the paper side layer and the interlacing pointson the machine side layer.

[0009] The degree of twinning of adjacent yarns can be described interms of the ratio of the difference of the distance (W) between one ofa specific twinned pair of yarns and the adjacent non-twinned yarn, andthe distance (N) between the twinned yarn pair, to the distance W. Thiscan be expressed as the ratio(W-N):W, or as a percentage (W-N)/W×100.

[0010] In a fabric with minimal twinning, this ratio would approach 0:1,or 0%; whereas in a highly twinned fabric, this ratio can be as high as1:2, or 50%. It has been found for the fabrics of this invention thatthe ratio can be reduced to at least 0.1:1, or 10%, and more preferablycan be reduced to between 0.05:1 and 0:1, or 5% to 0%.

[0011] The reduction of the twinning of the paper side weft yarns,together with the fact that all of the paper side layer weft yarnscontribute to the support of the paper making fibres, leads to a greaterregularity in the frame openings on the paper side surface of the paperside layer, and hence to a corresponding greater uniformity in the fibresupport. It is well known that the overall frame size and the framelength in the machine direction are important parameters in the designof forming fabrics, and these topics are discussed by Helle, Torbjorn,“Fibre Web. Support of the Forming Wire”, Tappi Journal, Vol. 71, No. 1(January 1988), pp. 112-117; and Johnson, D. B., “Retention and Drainageof Forming Fabrics”, Pulp & Paper Canada, Vol 85, pp. T167-172 (1984).The authors indicate that frame opening configurations have asignificant influence on the drainage of the incipient paper web, and onthe first pass retention characteristics of the forming fabric. It hasbeen found that greater cross machine direction support is achieved bythe use of designs having rectangular frame openings.

[0012] It has previously been considered that drainage problems indouble layer forming fabrics result from the use of weave patternsrequiring more than 8 sheds in the loom. For example, one aspect of suchproblems is noted in CPPA data sheet No. G18 (Rev. Nov. 1994), at page9. However, it has been found that suitable weave patterns can becreated using designs requiring 9 sheds or more, with advantageousresults, and without the expected disadvantages.

[0013] Consequently, it has been found that the lengths of the exposedfloats of the machine side layer weft yarns on the machine side surfaceof the machine side layer in a double layer forming fabric can beincreased. The resultant increased volume of weft material which issubjected to the abrasive forces of the machine can significantly extendthe operational life of the forming fabric.

[0014] The present invention therefore seeks to provide a double layerforming fabric for a paper making machine, having increased resistanceto machine side layer wear and abrasion. The invention provides forrelatively long machine side layer weft yarn floats in the machine sidesurface, which are exposed to the abrasive wear experienced by theforming fabric as it is running in contact with the various stationaryand moving elements in the forming section of the paper making machine.The invention also enables the use of larger diameter weft yarns thanhave previously been found feasible for use in double layer formingfabrics.

[0015] The present invention also seeks to provide a double layerforming fabric having an improved balance between water drainage andpaper solids retention. The invention provides substantially rectangularpaper side layer frame openings, having substantially the same width inthe machine direction. The regular spacing of the yarns forming theperimeters of the frame openings provides a high degree of uniformity ofsupport for the paper making fibres, so that the resulting sheet has asubstantially uniform appearance and structure.

[0016] The present invention still further seeks to provide a doublelayer forming fabric having a weave pattern which produces a substantialreduction in the twinning of the paper side layer weft yarns.

SUMMARY OF THE INVENTION

[0017] The present invention provides a double layer forming fabric fora paper making machine, woven to an overall repeating pattern, whichcomprises in combination

[0018] (a) paper side layer weft yarns;

[0019] (b) machine side layer weft yarns; and

[0020] (c) warp yarns,

[0021] wherein

[0022] (i) the paper side layer weft yarns interweave with the warpyarns in a first repeating weave pattern;

[0023] (ii) the machine side layer weft yarns interlace with the warpyarns in a second repeating weave pattern;

[0024] (iii) the fabric is woven in an overall repeating patternrequiring 3N-sheds, in which N is an integer and is at least 2;

[0025] (iv) for each warp yarn, the first repeating weave patternincludes interweaving locations comprising a first and a secondinterweaving point, each of said first and second interweaving pointsbeing separated by at least two paper side layer weft yarns;

[0026] (v) for each warp yarn, the second repeating weave patternincludes a series of interlacing points in which each interlaced machineside layer weft yarn also passes substantially below an interweavinglocation of paper side layer weft yarns and each adjacent warp yarn;

[0027] (vi) for each warp yarn, each of the first and secondinterweaving points with a paper side layer weft yarn is separated froman adjacent machine side layer interlacing point by at least two machineside layer weft yarns; and

[0028] (vii) a machine side surface of the machine side layer includesexposed machine side layer weft yarn floats having a float length Ldefined as L=3N-M, wherein M is an integer and is at least 1.

[0029] In the double layer forming fabrics of this invention, each warpyarn is intrinsic to the weave pattern in both the paper side layer andthe machine side layer of the fabric, so that each warp yarn contributesto the structural integrity and properties of both layers, particularlyin relation to consistency in the paper support, and allows for longweft yarn floats in the machine side layer, thus increasing theoperational life of the fabric.

[0030] Furthermore, in the double layer forming fabrics of thisinvention, each warp yarn follows an identical path, the weave patternfor each warp yarn being displaced from the weave pattern of adjacentwarp yarns by an identical predetermined number of paper side layer andmachine side layer weft yarns. Within each pattern repeat, the warp yarnpath includes interweaving locations comprising pairs of interweavingpoints with the paper side layer weft yarns, and interlacing points withthe machine side layer weft yarns, such that the interlacing points areapproximately centralized between the second interweaving point of apreceding interweaving location and the first interweaving point of thenext succeeding location. The displacement distance of one pair ofinterweaving points of one warp yarn from the preceding pair ofinterweaving points of the immediately preceding warp yarn, measured interms of the predetermined number of paper side layer and machine sidelayer weft yarns, is selected so that the machine side interlacingpoints on one warp yarn are located approximately beneath a portion ofan interweaving location on each adjacent warp yarn.

[0031] In addition, the warp yarns are arranged so that each warp yarninterlaces with the same machine side layer weft yarn as also interlaceswith either the second preceding or second subsequent warp yarn. Thisenables the use of designs including relatively long external weftfloats in the machine side surface of the machine side layer, byproviding stability to the long weft floats which, by the increased wearvolume in the machine side layer, contribute to the desired increasedoperational life of the fabric. Such designs include substantiallyregular frame openings on the paper side surface, which provides greateruniformity of the paper support.

[0032] Preferably, in all embodiments, each warp yarn has an internalfloat, between the machine side layer and the paper side layer, passingover at least two machine side layer weft yarns between each interlacingpoint with a machine side layer weft yarn and the immediately precedingand subsequent interweaving point in the paper side layer.

[0033] Preferably, the ratio of the number of paper side layer weftyarns to the number of machine side layer weft yarns is chosen frombetween 2:1 and 1:1, and more preferably the ratio is 2:1.

[0034] Preferably, the paper side layer weave pattern is selected from asatin weave design, or a twill or a broken twill weave design. Morepreferably the paper side layer weave pattern is a 3N×6N design, where3N is the number of sheds. Where the ratio of the number of paper sidelayer weft yarns to the number of machine side layer weft yarns is 2:1,the machine side layer weave pattern would thus preferably be a 3N×3Ndesign.

[0035] In the following description and claims, certain terms have thefollowing meanings:

[0036] The term “paper side layer” refers to the layer of weft and warpyarns in the double layer forming fabric onto which the stock isdeposited, and the associated term “paper side surface of the paper sidelayer” refers to the exposed surface of the paper side layer whichdirectly supports the incipient paper web.

[0037] The term “machine side layer” refers to the layer of weft andwarp yarns in the double layer forming fabric which is in contact withthe support means of the paper making machine, and the associated term“machine side surface of the machine side layer” refers to the exposedsurface of the machine side layer which is in direct contact with thestationary and rotating elements of the machine.

[0038] The term “machine direction” or “MD” refers to a line parallel tothe direction of travel of the forming fabric when in use on the papermaking machine, and the associated term “cross machine direction” or“CD” refers to a direction transverse to the machine direction.

[0039] The term “frame” refers to the substantially rectangular areadefined by the longitudinal axis of four interwoven yarns in the paperside surface of the paper side layer of a forming fabric. The associatedterm “frame size” refers to the size determined by measurement from fourselected yarns which define in plan view a distinct frame. This term issynonymous with the term “top surface open area” as used in CPPA DataSheet No. G18 (Rev. Nov. 1994), at page 3. The associated term “frameopening” refers to the actual open area in between the yarns within agiven frame in the paper side surface of the paper side layer of thefabric.

[0040] The term “fibre support index” refers to a calculation madeaccording to the method described by Beran and summarized in CPPA datasheet No. G-18 (Rev. Nov. 1994) at page 4; it provides an indication ofthe level of support given to the incipient paper web by the formingfabric. The method is further detailed in Helle, Torbjorn, “Fibre WebSupport of the Forming Wire”, Tappi Journal, supra at p. 115.

[0041] The term “interlace” refers to a locus at which a specific warpyarn wraps about a machine side layer weft yarn; the associated term“interweave” refers to a locus at which a specific warp yarn wraps abouta paper side layer weft yarn.

[0042] The term “float” refers to a yarn which passes over a group ofother yarns without interweaving or interlacing with them; theassociated term “float length” refers to the length of a float, whichcan be expressed as a number indicating the number of yarns passed over.

[0043] The term “internal float” refers to a float which passes betweenthe adjacent surfaces of the machine side layer and the paper sidelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The invention will now be described by way of reference to thedrawings, in which:

[0045] FIGS. 1 to 18 inclusive are sequential schematic cross-sectionalviews of a first embodiment of the invention, showing the paths of eachsuccessive warp yarn in one repeat of the forming fabric weave pattern;

[0046]FIG. 19 depicts the paper side layer of the first embodiment ofthe invention;

[0047]FIG. 20 depicts the machine side layer of the first embodiment ofthe invention;

[0048]FIG. 21 is a weave diagram of the first embodiment of theinvention;

[0049]FIG. 22 is a weave diagram of a second embodiment of theinvention, also showing the path of one warp yarn in one repeat of theforming fabric weave pattern; and

[0050]FIG. 23 is a weave diagram of a third embodiment of the invention,also showing the path of one warp yarn in one repeat of the formingfabric weave pattern.

DETAILED DESCRIPTION OF THE INVENTION

[0051] Referring first to FIGS. 1 to 18, these figures taken togethershow the path of each of eighteen single successive warp yarns 140 ofthe overall fabric repeat pattern of a first embodiment of the formingfabric 100 of the invention. The warp yarns 140 in consecutive figuresare identified consecutively as warp yarn A, warp yarn B, warp yarn C upto and including warp yarn R. In each of FIGS. 1 to 18, the weft yarns120 in the paper side layer 102 are shown in cross-section as the upperlayer, and the weft yarns 130 in the machine side layer 104 are shown incross-section as the lower layer. The two sets of weft yarns 120 and 130are numbered from 1 to 54.

[0052] It can be seen that each warp yarn 140 follows an identical path,forming in one repeat of the paper side layer 102 weave pattern twointerweaving locations 105 and 107, each comprising two interweavingpoints 106 and 108, and 110 and 112 (FIG. 1), and in one repeat of themachine side layer 104 weave pattern two interlacing points 114 and 116,shown for warp yarn A as 114 a, 116 a, for warp yarn B as 114 b, 11,6 betc. In each repeat, reading from the left of the figures, interlacingpoint 114 follows interweaving point 108 and precedes interweaving point110.

[0053] Referring to the path of warp yarn A shown in FIG. 1, at eachinterweaving location 105, each pair of interweaving points 106, 108 and110, 112 respectively is separated by, and thus forms an internal warpfloat of, two paper side layer weft yarns 120, seen in FIG. 1 as paperside layer weft yarns 2 and 4, and 29 and 31.

[0054] Referring to FIGS. 1, 2 and 3, showing the paths of warp yarns A,B and C respectively, it will be seen that machine side layer weft yarn3, which interlaces with warp yarn B at interlacing point 116 b, alsopasses directly under the first interweaving location 105 of warp yarnA, which occurs at paper side layer weft yarns 1 and 5, and passessubstantially under the first interweaving location 107 of warp yarn C,which occurs at paper side layer weft yarns 4 and 8.

[0055] Similarly, machine side layer weft yarn 33, which interlaces withwarp yarn B at interlacing point. 114 b, passes directly under thesecond interweaving location 105 of warp yarn C, which occurs at paperside layer weft yarns 31 and 35, and also passes substantially under thesecond interweaving location 107 of warp yarn A, which occurs at paperside layer weft yarns 28 and 32.

[0056] It can further be seen, referring to FIGS. 1 to 18, that for eachof warp yarns A, B, C, and each succeeding warp yarn D to R, each firstinterlacing point 114 a, 114 b, 114 c etc. is separated from both theimmediately preceding interweaving point 108 and each succeedinginterweaving point 110 on the same warp yarn by at least three machineside layer weft yarns 130. For example, in FIG. 1, first interlacingpoint 114 a at machine side layer weft yarn 18 is separated from firstinterweaving point 108 at paper side layer weft yarn 5 by machine sidelayer weft yarns 6, 9, 12 and 15, and from second interweaving point 110at paper side layer weft yarn 28 by machine side layer weft yarns 21, 24and 27. Similarly, each second interlacing point 116 a, 116 b, 116 cetc. is separated from the immediately preceding interweaving point 112and each succeeding interweaving point 106 by at least three machineside layer weft yarns 130.

[0057] Still referring to FIGS. 1 to 18, it will further be noted thateach machine side layer weft yarn 130 has an external float length inthe machine side surface of the machine side layer of 15 warp yarns 140.For example, the machine side layer weft yarn 18 has an interlacingpoint 114 a with warp yarn A, and a second interlacing point 116 c withwarp yarn C, but has no further interlacing points in the machine sidelayer weave pattern repeat, thus passing below and on the machine sideof each of the fifteen warp yarns D to R. Similarly, machine side layerweft yarn 42 has an interlacing point 116 a with warp yarn A, and asecond interlacing point 114q with warp yarn Q, but no furtherinterlacing points in the machine side layer weave pattern repeat, thuspassing below and on the machine side of each of the fifteen warp yarnsB to P.

[0058] It can further be seen that for any group of three adjacent warpyarns 140, in one repeat of the overall weave pattern, the first andthird warp yarns 140 each interlace once, i.e., at either interlacingpoint 114 or interlacing point 116, but not both, with a common machineside layer weft yarn 130. Thus warp yarns A and C at their respectiveinterlacing points 114 a and 116 c are separated by warp yarn B.Similarly, warp yarns Q and A at their respective interlacing points 114q and 116 a are separated by warp yarn R. The effect of this aspect ofthe second repeating weave pattern can be seen in FIG. 20, whereinterlacing point 114 is indicated.

[0059] One result of this pattern of pairs of interlacing points 114 or116 is an increase in the crimp differential of the machine side layerweft yarns 130 at these points, which causes them to bow outwards awayfrom the machine side surface of the machine side layer 104, thusincreasing their prominence. This results in an increase in theavailable wear volume of the machine side layer weft yarns 130 exposedto abrasion, thus increasing the operational life of the fabric.

[0060] Still referring to FIGS. 1 to 18, it will be seen that eachsuccessive warp yarn 140 follows an identical path, the pattern of whichis displaced from the pattern of the immediately preceding warp yarn 140by the same number of paper side layer weft yarns 120, and the samenumber of machine side layer weft yarns 130. For example, referring toFIGS. 1 to 4, the first interweaving point 106 of warp yarn A is withpaper side layer weft yarn 1, and the first subsequent interweavingpoint 106 of warp yarn B is with paper side layer weft yarn 16. Thefirst subsequent interweaving point 106 of warp yarn C is with paperside layer weft yarn 31, and the first subsequent interweaving point 106of warp yarn D is with paper side layer weft yarn 46. Thus in this firstembodiment, the displacement can be seen to comprise 10 paper side layerweft yarns 120, the subsequent interweaving point 106 being on the tenthpaper side layer weft yarn 120 from the interweaving point 106 on thepreceding warp yarn 140. Similarly, the displacement also comprises fivemachine side layer weft yarns 130, each interlacing point 114 or 116being on the sixth machine side layer weft yarn 130 from the respectiveinterlacing point 114 or 116 on the preceding warp yarn 140.

[0061] It can further be seen from FIGS. 1 to 18 that each interlacingpoint 114 in the machine side layer 104 is located respectivelysubstantially below a central location 115 in the paper side layer 102between the second interweaving point 108 and the next followinginterweaving point 110. Similarly, each interlacing point 116 in themachine side layer is located substantially below a central locationbetween the second interweaving point 112 and the next followinginterweaving point 106. In the embodiment shown in FIGS. 1 to 18, thefirst central location 115 is separated from interweaving point 108 byeight paper side layer weft yarns 120, and from interweaving point 110by six paper side layer weft yarns 120. The second central location 117is separated from interweaving point 112 by six paper side layer weftyarns 120, and from the next following interweaving point 106 by eightpaper side layer weft yarns 120. This arrangement of interlacing andinterweaving points is constant for each of the warp yarns A to R inFIGS. 1 to 18.

[0062] Still referring to FIGS. 1 to 18, it can further be seen that inthe repeating weave pattern of the paper side layer 102, theinterweaving points are aligned so that for each interweaving location105 or 107 on a selected warp yarn 140, comprising a pair ofinterweaving points 106, 108 or 110, 112, one interweaving point on eachof the second preceding and second subsequent warp yarns 140 is locatedon a paper side layer weft yarn 120 between the paper side layer weftyarns with which the selected warp yarn 140 interweaves. For example,considering warp yarn C in FIG. 3 as being the selected warp yarn 140,the first interweaving location 105 comprises interweaving points 106and 108 at paper side layer weft yarns 4 and 8 respectively. The secondpreceding warp yarn 140 would be warp yarn A (FIG. 1), which has aninterweaving point 108 with paper side layer weft yarn 5. The secondsubsequent warp yarn 140 is warp yarn E (FIG. 5), which has aninterweaving point 106 with paper side layer weft yarn 7. Similarly, forthe second interweaving location 107 on warp yarn C (FIG. 3), theinterweaving points 110, 112 are with paper side layer weft yarns 31 and35. The corresponding interweaving point 112 on warp yarn A (FIG. 1) iswith paper side layer weft yarn 32, and the corresponding interweavingpoint 110 with warp yarn E (FIG. 5) is with paper side layer weft yarn34. A similar pattern can be identified in considering the interweavingpoints 106, 108 and 110, 112 on each warp yarn 140.

[0063] It can further be seen from FIG. 21 that this spatialrelationship of interweaving points 106, 108, and 110, 112 on successivealternate warp yarns 140 comprises a series of substantially rhomboidbracing zones 142, of identical configuration. Two examples are shown inFIG. 21, in which warp yarns 1, 3 and 5 correspond with warp yarns A, Cand E in FIGS. 1, 3 and 5.

[0064] The effect of these bracing zones 142 is to provide a bracingeffect on the paper side layer weft yarns 120 at each interweavinglocation 105 and 107, which has been found to have the advantage offurther reducing any tendency to twinning of pairs of paper side layerweft yarns 120.

[0065] As discussed above, the degree of twinning of pairs of yarns inthe fabrics of the present invention can be reduced so that the ratio ofthe distance between twinned yarns and adjacent non-twinned yarns isless than 0.1:1, or 10% and is preferably between 0.05:1 and 0:1, or 5%to zero.

[0066] Referring to FIGS. 1 to 18, and FIG. 21, and as already notedabove, the first embodiment thus comprises a forming fabric 100 havingan overall repeating pattern requiring eighteen sequential warp yarnpaths, and having a first repeating weave pattern, in the paper sidelayer 102, comprising 36 paper side layer weft yarns 120. The secondrepeating weave pattern, in the machine side layer 104, over the samedistance comprises 18 machine side layer weft yarns 130. Thus theforming fabric of this embodiment can be seen as having a firstrepeating weave pattern of 3N by 6N, and a second repeating weavepattern of 3N by 3N. For the fabric of this embodiment, it can thus beseen that 3N is 18, and N=6.

[0067] In the first embodiment, shown in FIGS. 1 to 18 and 21, the ratioof the paper side layer weft yarns 120 to the machine side layer weftyarns 130 is 2:1.

[0068] The machine side layer weft yarns 130 are not necessarily of thesame diameter as, and are preferably of a larger diameter than, thepaper side layer weft yarns 120. Wilson, in U.S. Pat. No. 6,112,774,suggests that each CD yarn in the machine side layer may require to besubstantially aligned with a CD yarn in the paper side layer. However,it has been found that although the 18 machine side layer weft yarns 130occupy the same distance in the machine direction as the 36 paper sidelayer weft yarns 120, none of the machine side layer weft yarns isrequired to be aligned specifically with any of the paper side layerweft yarns 120.

[0069] Referring to FIG. 19, showing the paper side surface of the paperside layer 102, it can be seen that the first repeating weave patternresults in regular frame openings 150. As discussed above, this featurehas been found to contribute to improved drainage properties of thepaper side layer of a double layer forming fabric. It can further beseen that the substantially rectangular openings 150 are to some extentlonger in the CD than in the MD. As discussed above, this featurecontributes to CD support of the paper making fibres, which arepredominantly MD oriented in the incipient paper web. The Beran's “b”figure used in the calculation of the fibre support index, as determinedby the method described in the CPPA Data Sheet, noted above, for thefabrics of this invention is at least 0.8, and is more preferablybetween 0.8 and 1.0, and most preferably is 1.0, indicating that all ofthe paper side layer weft yarns 120 contribute to supporting thepapermaking fibres.

[0070] Further referring to FIG. 19, a typical interweaving location105, of paper side layer weft yarns 120 and warp yarns 140, comprisesinterweaving points 106 and 108. A bracing zone 142 is also shown.

[0071] Referring to FIG. 20, showing the machine side surface of themachine side layer 104, the interlacing points 114 and 116 of machineside layer weft yarns yarns 140 can be seen. By following the path ofeach warp yarn 140 on either side of an interlacing point 114 or 116, itcan further be seen that each two warp yarns 140 which appear to beadjacent at their respective interlacing points 114 and 116 are in factseparated by a third warp yarn 140.

[0072] A second embodiment of the double layer forming fabric of theinvention is shown in FIG. 22. In this embodiment, the paper side layer102 and the machine side layer 104 are each woven to a 9-shed satinweave pattern, for which N=3. The weave diagram of FIG. 22 shows onerepeat in the MD and two repeats in the CD of both the paper side layerand machine side layer weave patterns. As can be more clearly seen fromthe diagram showing the path of one warp yarn 140, in each repeat of therepeating weave pattern in the paper side layer 102, there is a singleinterweaving location 105, at which each pair of interweaving points106, 108 is separated by two paper side layer weft yarns 120. Forexample warp yarn 1 interweaves with paper side layer weft yarns 1 and5, which are separated by paper side layer weft yarns 2 and 4. However,in this embodiment, at each interlacing point 114, each warp yarn 140interlaces with an adjacent pair of machine side layer weft yarns, shownin the warp yarn path diagram of FIG. 22 as machine side layer weftyarns 15 and 18.

[0073] It can further be seen from FIG. 22 that for any three warp yarns140, the first and third warp yarn 140 interlace with a common machineside layer weft yarn 130. Thus, for example, warp yarn 1 interlaces withmachine side layer weft yarns 15 and 18, and warp yarn 3 interlaces withmachine side layer weft yarns 18 and 21. Similarly, warp yarn 2interlaces with machine side layer weft yarns 3 and 6, and warp yarn 4interlaces with machine side layer weft yarns 6 and 9. As has alreadybeen noted in relation to the first embodiment, this pattern of doubleinterlacing points 114 has been found to increase the crimp differentialof the machine side layer weft yarns 130, causing them to become moreprominent on the machine side surface of the machine side layer and,together with the effects of the longer float lengths of the machineside layer weft yarns 130, results in a corresponding increase in theoperational life of the fabric.

[0074] It has been found that the interlacing of each warp yarn 140 withtwo adjacent machine side layer weft yarns 130 in this embodimentprovides the additional advantage that a larger diameter yarn can beused for the machine side layer weft yarns, which can further increasethe operational life of the fabric.

[0075] In this embodiment, in a similar manner to the first embodiment,the repeating weave pattern in the paper side layer 120 also includesbracing zones 142. For example, again referring to FIG. 22, warp yarn 3interweaves with paper side layer weft yarns 4 and 8, warp yarn 1interweaves with paper side layer weft yarn 5, and warp yarn 5interweaves with paper side layer weft yarn 7.

[0076] A third embodiment of the double layer forming fabric of theinvention is shown in FIG. 23. In this embodiment, the paper side layer102 and the machine side layer 104 are each woven to a 9-shed satinweave pattern, for which N=3. The weave diagram of FIG. 23 shows onerepeat in the MD and two repeats in the CD of both the paper side layerand machine side layer repeating weave patterns. In this embodiment, ineach repeating weave pattern in the paper side layer 102, there is asingle interweaving location 105, at which each pair of interweavingpoints 106, 108 is separated by two paper side layer weft yarns 120. Forexample, in FIG. 23, warp yarn 1 interweaves with paper side layer weftyarns 2 and 6, which are separated by paper side layer weft yarns 3 and5.

[0077] In this embodiment, in a similar manner to the first and secondembodiments, the repeating weave pattern in the paper side layer 120also includes bracing zones 142. For example, again referring to FIG.23, warp yarn 3 interweaves with paper side layer weft yarns 5 and 9,warp yarn 1 interweaves with paper side layer weft yarn 6, and warp yarn5 interweaves with paper side layer weft yarn 8. It can be seen that thepaper side layer 102 presents a uniform support surface for theincipient web, and has a fibre support index of approximately 1.

[0078] In this embodiment, the pattern of interlacing points 114 differsfrom that of the first two embodiments in that it does not include theinterlacing of each of a first and third warp yarns 140 with a commonmachine side layer weft yarn 130. The pattern of this embodiment mayrequire a somewhat reduced maximum diameter which can be used for themachine side layer weft yarns 130 than can be used for the first orsecond embodiments. However, any restriction on the extended operationallife of the fabric can be offset by the increased wear potential whichis derived from the float lengths of 8 for the machine side layer weftyarns 130.

[0079] The warp yarns 140 can be made of any suitable polymer material,and preferably have a substantially circular cross-section, althoughoval, elliptical and other geometric shaped cross-sections may be used.The dimensions of the warp yarns 140, the paper side layer weft yarns120 and the machine side layer weft yarns 130 can be selected dependingon factors including the intended end use, particularly the intendedpaper grade.

[0080] Experimental fabrics woven according to the various embodimentsof the invention utilized machine side layer weft yarns 130 having acircular cross-section, and a diameter of 0.45 mm. These were eitherpolyethylene terephthalate (PET), or alternating polyester and nylon-6or nylon-6/6. Wear resistant yarns comprised of polymer blends of PETand thermoplastic polyurethane such as are disclosed by Bhatt et al, inU.S. Pat. No. 5,502,120, were also found effective in increasing thewear potential of the forming fabric of the invention. Yarn diametersranging from 0.40 mm to 0.50 mm have been found to provide satisfactoryresults.

[0081] For the paper side layer weft yarns 120, a PET polyester was usedhaving a circular cross-section and a diameter of 0.26 mm, but theresults suggest that a range of 0.17 mm to at least 0.26 mm would givesatisfactory results.

[0082] For the warp yarns 140, high modulus yarns were found to beparticularly suitable, such as those comprised of polyethylenenaphthalate (PEN). These yarns have a circular cross-section and adiameter ranging from 0.20 mm to 0.25 mm. Yarns made from thesematerials tend to retain their crimp particularly well following weavingand heatsetting, and the resulting fabrics exhibit a reduced propensityto stretch. Due to their high modulus, it is possible to use smalleryarns than comparable yarns of PET, while retaining comparable physicalproperties. This provides the possibility of using warp yarns 140 of PENto reduce the warp fill and thus allow for more rapid drainage of waterfrom the incipient web, if this is desired in a particular situation.

[0083] Those of skill in the art may vary the yarn sizes and materialsused in the fabrics of the invention so as to accommodate the prevailingconditions and parameters of use in the particular paper making machine.

[0084] The fabrics of the invention will generally be woven flat, andsubsequently cut and seamed in order to provide the required endlessloop of fabric.

What is claimed is:
 1. A double layer forming fabric for a paper makingmachine, woven to an overall repeating pattern, which comprises incombination: (a) paper side layer weft yarns; (b) machine side layerweft yarns; and (c) warp yarns, wherein (i) the paper side layer weftyarns interweave with the warp yarns in a first repeating weave pattern;(ii) the machine side layer weft yarns interlace with the warp yarns ina second repeating weave pattern; (iii) the fabric is woven in anoverall repeating pattern requiring 3N sheds, in which N is an integerand is at least 2; (iv) for each warp yarn, the first repeating weavepattern includes interweaving locations comprising a first and a secondinterweaving point, each of said first and second interweaving pointsbeing separated by an internal warp float of at least two paper sidelayer weft yarns; (v) for each warp yarn, the second repeating weavepattern includes a series of interlacing points in which each interlacedmachine side layer weft yarn also passes substantially below aninterweaving location of paper side layer weft yarns and each adjacentwarp yarn; (vi) for each warp yarn, each of the first and secondinterweaving points with a paper side layer weft yarn is separated froman adjacent machine side layer interlacing point by at least two machineside layer weft yarns; and (vii) a machine side surface of the machineside layer includes exposed machine side layer weft yarn floats having afloat length L defined as L=3N-M, wherein M is an integer and is atleast
 1. 2. A forming fabric according to claim 1 wherein for each warpyarn, each of the first and second interweaving points with a paper sidelayer weft yarn is separated from an adjacent machine side layerinterlacing point by at least three machine side layer weft yarns.
 3. Aforming fabric according to claim 1 or claim 2 wherein each warp yarnfollows an identical path.
 4. A forming fabric according to claim 3wherein each successive interlacing point in the machine side layer islocated substantially under a central location between the secondinterweaving point of each interweaving location and the firstinterweaving point of an adjacent subsequent interweaving location.
 5. Aforming fabric according to claim 4, wherein for each warp yarn, thenumber of machine side layer weft yarns between each interlacing pointand an immediately preceding interweaving point differs from the numberof machine side layer weft yarns between the same interlacing point andan immediately subsequent interweaving point.
 6. A forming fabricaccording to claim 1, 2, 3 or 4, wherein the ratio of the number ofpaper side layer weft yarns to the number of machine side layer weftyarns is from 2:1 to 1:1.
 7. A forming fabric according to claim 6,wherein the ratio is 2:1.
 8. A forming fabric according to claim 1 orclaim 2, wherein the first repeating weave pattern is a 3N by 6Npattern, and the second repeating weave pattern is a 3N by 3N pattern.9. A forming fabric according to claim 1 or claim 2, wherein at eachmachine side layer interlacing point each warp yarn interlaces with apair of adjacent machine side layer weft yarns.
 10. A forming fabricaccording to claim 1 or claim 2, wherein the material of construction ofthe machine side layer weft yarns comprises a wear resistant polymer.11. A forming fabric according to claim 10, wherein the wear resistantpolymer is selected from polyethylene terephthalate, nylon-6, nylon-6/6and polymer blends.
 12. A forming fabric according to claim 11, whereinthe material of construction of the machine side layer weft yarns is apolymer blend and comprises polyethylene terephthalate and athermoplastic polyurethane.
 13. A forming fabric according to claim 1 orclaim 2, wherein the material of construction of the warp yarns includesa polymer selected from polyethylene terephthalate and polyethylenenaphthalate.
 14. A forming fabric according to claim 1 or claim 2,wherein a Beran's “b” figure for calculation of a fibre support index[as described in CPPA Data Sheet G-18 (Rev. Nov. 1994)] in respect ofthe paper side layer weft yarns is in the range from 0.8 to 1.0.
 15. Aforming fabric according to claim 14, wherein the Beran's “b” figure is1.0.